The primary function of eyeglasses and contact lenses is in having the optical power of those lenses compensate for the loss of acuity (myopia) or excessive acuity (hyperopia) of the patient. Traditional vision acuity tests have used static optotypes as displays of printed or projected characters, objects, or shapes. Numerous patterns, configurations, and methods for static optotypes have been proposed for testing acuity based upon the ability of a subject to distinguish these various shapes, sizes, contrasts, and colors in tests such as Snellen charts, tumbling “E” arrays (static images of the letter “E” where the static image is also rotated 90 degrees, 180 degrees, and 270 degrees for discernment), Landolt “C” charts, and so on. Certain prior art vision testing patterns use periodic images, such as disks, rectangles, diamonds, etc.; others are quasi-periodic, such as tri-bar, and small checkerboard designs.
While the Landolt “C” chart is the clinical standard for acuity, the familiar Snellen eye testing chart as developed in 1862 using large, black, serifed letters on a white background is the test frequently used for determining visual acuity. The concept of these charts to verify acuity is based upon the patient seeing patterns such as letters or printed images on those charts, or as an image made visible by the reflection or projection of scattered light. Snellen's standard is that a person should be able to see and identify a 3.5 inch letter at a 20 foot distance (that ratio being consistent regardless of its use in the “English” or Metric system). This, however, assumes that the light is sufficiently bright so that the patient is able to identify the image and that there is sufficient contrast between the image and the background. A disadvantage of the Snellen type images is that even defocused letters can still be partially recognized by their blur patterns. Much time is thus wasted as the patient, whose eyes are being tested, attempts to guess the letter. The design of the Snellen chart is further complicated by each letter having a different degree of recognizability and by the tendency of the patient to strain to perceive coherency when trying to identify the letters. Additionally, the precision of the Snellen test is incumbent upon the individual identifying three of the five letters displayed at a 20 foot distance. Identification of less than three letters indicates insufficient refraction; however identification of more than three letters is actually over-refraction. These factors create a potential for both over-refraction and over-compensation.
Numerous other studies have shown difficulties with generating appropriate projected Snellen images as based upon technology developed in 1922 and updated in 1948. Projection systems are typically dependent upon a darkened room to enhance the contrast of the Snellen images, thereby creating a “contrast sensitivity” which may compromise actual acuity. Projection systems also inherently create a “fuzzy” image resulting from the mechanics of the diffraction of light waves, thus decreasing the accuracy of the perceived refraction.
Numerous attempts have been made to utilize recent technology to reproduce the Snellen test concept on computers or other devices with a high contrast display. Images from electronically generated characters, such as those from a cathode ray tube (CRT) or liquid crystal display (LCD), produce images that are distinctly sharper and less confusing than print or projected images. Such displays eliminate the inherent fuzziness of the Snellen test, but they still do not eliminate the tendency to misperceive letters and images inherent in focusing on significantly distant static images.